lisa_shell: don't fail when running "lisa-update" if submodules aren't synced
lisa_shell: don't fail when running "lisa-update" if submodules aren't synced

"lisa-update all" can fail if submodules are not pointing at the current
tracked head.  This can happen if you have worked on a submodule's branch.

This happens because "git diff-index" reports changes to submodules as
uncommitted changes, which is not what we want.  Add
"--ignore-submodules" to its invocation to make it ignore changes to
submodules.

Note that this won't report uncommitted changes to a submodule that
can't be merged, but that is not an issue.  The purpose of this
"git diff-index" is to make sure we can run the subsequent "git pull" and
that doesn't affect the submodules.  If you have outstanding
uncommitted changes in one of the submodules, then
_lisa-update-submodules will fail (or rather "git submodule update" will
fail) but that is the correct behaviour: lisa-update all has updated all
it could (lisa, and as many modules as possible) and the submodule
command that fails and asks you to "Please, commit your changes or stash
them" because it is "Unable to checkout $COMMIT_ID in submodule path
'libs/trappy'".

credp/lisa,ARM-software/lisa,ARM-software/lisa,credp/lisa,arnoldlu/lisa,credp/lisa,bjackman/lisa,ARM-software/lisa,bjackman/lisa,mdigiorgio/lisa,arnoldlu/lisa,mdigiorgio/lisa,credp/lisa,joelagnel/lisa,joelagnel/lisa,ARM-software/lisa